The book is narrated by
Patroclus, the son of King
Menoetius. He is presented as a potential suitor to
Helen of Troy. He is then obliged to take a blood oath in defense of her marriage to
Menelaus. After Patroclus accidentally kills the son of one of his father's nobles after said son attacked him, he is exiled to
Phthia, where he meets
Achilles, the only son of Phthia's king,
Peleus, and the sea nymph
Thetis. They become close friends, and Patroclus develops feelings for Achilles. Convinced that a mortal of low status is an unsuitable companion for her son, Thetis attempts to separate the pair by sending Achilles to train under
Chiron. Patroclus thwarts the plan by running away to join Achilles on
Mount Pelion. As he matures, he struggles to hide his deepening attraction to the demigod. Achilles reciprocates and they have sex, establishing their romantic relationship despite Thesis' attempts to separate them. Soon the
Mycenaean king,
Agamemnon, calls on the various
Achaeans to join his military campaign against
Troy, whose prince
Paris has kidnapped his brother Menelaus's wife, Helen. As a prophecy foretells that Achilles will die in Troy after the death of the Trojan prince
Hector, Thetis hides Achilles on
Skyros in the guise of a woman in the court of King
Lycomedes; she forces him to marry Lycomedes's daughter
Deidamia, who later bears Achilles's son
Neoptolemus, also known as
Pyrrhus. Patroclus follows Achilles to Skyros, where they live until they are discovered by
Odysseus and
Diomedes. Patroclus is obligated to join the war in Troy as a result of his blood oath, while Achilles joins after vowing that he will never fight Hector in order to avoid his prophesied death. Upon joining the Achaean forces, tensions escalate between Achilles and Agamemnon: first when Agamemnon sacrifices his daughter
Iphigenia in order to appease
Artemis and later when Achilles takes the Trojan woman
Briseis as a war prize to save her from Agamemnon. However, out of sensitivity, Achilles largely avoids interacting with Briseis directly because he had killed the men in her family, but she and Patroclus develop a close friendship verging on romance, ultimately settling into a devoted sibling relationship. After nine years,
Chryseis is claimed by Agamemnon. Soon after, her father,
Chryses, tries to pay for her release, which Agamemnon refuses. Chryses appeals to
Apollo, who unleashes a plague that decimates the Achaeans; when Agamemnon refuses Achilles's demand to return Chryseis, he doubles down by blaming Achilles for the war's length, by his unwillingness to face and kill Hector. As punishment, he orders that Briseis be taken from Achilles and brought to him, which offends Achilles, who vows to remove himself and his army from the fight until this slight at his honor is repaired. Achilles begs his mother to avenge him by punishing the Archean soldiers who had wronged him and precipitate the Greeks' need for him; Thetis convinces
Zeus to tip the war in favor of the Trojans so that the Achaeans will regret having antagonized Achilles, and the Achaeans suffer from a deadly plague. Tensions flare between Achilles and Patroclus when Achilles refuses to accept a private arrangement where Briseis is returned to him, along with valuable gifts. He stubbornly demands a public apology, refusing to come to the aid of the Greeks, who are on the verge of defeat. Patroclus, who has grown close to the soldiers as a field medic and sympathizes with their losses, attempts and fails to convince Achilles to rejoin the battle. Instead, Patroclus impersonates Achilles by donning his armor and leads his men into battle; the offensive forces a Trojan retreat. During the battle, Patroclus killed
Sarpedon and
Apollo causes Patroclus to reveal himself. Patroclus is killed by Hector, and his body is brought to Achilles. Achilles grieves along with Briseis and demands Patroclus's ashes be mixed with his own when he dies. Having lost his will to live, Achilles returns to battle and kills Hector to avenge Patroclus. After he is in turn killed by Paris with the divine help of Apollo, his ashes are mixed with Patroclus's, per his request, and are buried. Neoptolemus comes to take Achilles's place and kills Briseis when she refuses his advances and reveals Achilles and Patroclus's relationship. The Achaeans erect a tomb for Achilles and Patroclus but do not inscribe Patroclus's name at the behest of Neoptolemus. Patroclus's
shade is thus unable to pass into the
underworld and is bound to the tomb. After the war, Thetis returns and grieves for Achilles. She and Patroclus share memories of the fallen hero, and Thetis relents, writing Patroclus's name upon the tomb. Patroclus is now able to pass into the afterlife, where he and Achilles reunite. ==Development and publication==